We investigate nonclassical Stokes-operator variances in continuous-wave polarization-squeezed laser light generated from one and two optical parametric amplifiers. A general expression of how Stokes-operator variances decompose into two-mode quadrature operator variances is given. Stokes parameter variance spectra for four different polarization-squeezed states have been measured and compared with a coherent state. Our measurement results are visualized by three-dimensional Stokes-operator noise volumes mapped on the quantum Poincaré sphere. We quantitatively compare the channel capacity of the different continuous-variable polarization states for communication protocols. It is shown that squeezed polarization states provide 33% higher channel capacities than the optimum coherent beam protocol.

I describe the use of techniques based on composite rotations to combat systematic errors in controlled phase gates, which form the basis of two-qubit quantum logic gates. Although developed and described within the context of nuclear magnetic resonanace quantum computing these sequences should be applicable to any implementation of quantum computation based on Ising couplings. In combination with existing single-qubit gates this provides a universal set of robust quantum logic gates.

We introduce and study the concept of a reversible transfer of the quantum state of two internally-translationally entangled fragments, formed by molecular dissociation, to a photon pair. The transfer is based on intracavity stimulated Raman adiabatic passage and it requires a combination of processes whose principles are well established.

The $1s2s{}^3{S}_1\to 1s2p{}^3{P}_{1,2}$ transition in ${}^{6,7}{\mathrm{Li}}^{+}$ was investigated by exciting a 5–6-keV ion beam with a nearly copropagating ring-dye laser beam. The laser scan was calibrated using an electro-optic modulator to generate frequency sidebands. Results for the ${}^6{\mathrm{Li}}^{+}$ hyperfine splittings are an order of magnitude more precise than existing data in the literature and the ${}^7{\mathrm{Li}}^{+}$ results resolve an 11-MHz (17σ) discrepancy for the ${}^3{P}_{1–2}$ fine-structure interval as determined by two previous experiments. The results for 12 (all 13) measured hyperfine intervals in ${}^{6,7}{\mathrm{Li}}^{+}$ are within 2σ (3σ) of the latest Hylleraas variational calculations.

Collisional effects can have strong influences on the population densities of excited states in gas discharges at elevated pressure. The knowledge of the pertinent collisional coefficient describing the depopulation of a specific level (quenching coefficient) is, therefore, important for plasma diagnostics and simulations. Phase-resolved optical emission spectroscopy (PROES) applied to a capacitively coupled rf discharge excited with a frequency of 13.56 MHz in hydrogen allows the measurement of quenching coefficients for emitting states of various species, particularly of noble gases, with molecular hydrogen as a collision partner. Quenching coefficients can be determined subsequent to electron-impact excitation during the short field reversal phase within the sheath region from the time behavior of the fluorescence. The PROES technique based on electron-impact excitation is not limited—in contrast to laser techniques—by optical selection rules and the energy gap between the ground state and the upper level of the observed transition. Measurements of quenching coefficients and natural fluorescence lifetimes are presented for several helium $(3\mathrm{}{}^{1}S,4\mathrm{}{}^{1}S,3\mathrm{}{}^{3}S,3\mathrm{}{}^{3}P,4\mathrm{}{}^{3}S),$ neon ${(2p}_1{,2p}_2{,2p}_4{,2p}_6),$ argon ${(3d}_2{,3d}_4{,3d}_1^{\prime }$ and ${3d}_3),$ and krypton ${(2p}_1{,2p}_5)$ states as well as for some states of the triplet system of molecular hydrogen.

The angular and impact-energy dependence of the characteristic K x radiation of argon produced from collisions of electrons with 10.0–24.0 keV energies has been investigated. The radiation is found to show a net linear polarization of up to $(17\mathrm{}\pm 6)\%$ in the investigated range of impact energy. This anisotropy is believed to arise from the strong polarization of $K\alpha$ satellite lines of argon produced by the simultaneous decay of a single K- and multiple L-shell vacancy states created in the collisions. Also, the behavior of the net polarization of K radiation as a function of impact energy in the range of 10.0 to 24.0 keV has been studied. Over the range of impact energies, the polarization is found to show a nearly constant value within the experimental uncertainty.

We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte Carlo solution of the Liouville master equation for the joint probability density of the ionic motion and the electronic population of the projectile and the target surface. It includes single as well as double particle-hole (de)excitation processes and incorporates electron correlation effects through the conditional dynamics of population strings. The input in terms of elementary one- and two-electron transfer rates is determined from classical trajectory Monte Carlo calculations as well as quantum-mechanical Auger calculations. For slow projectiles and normal incidence, the ionic motion depends sensitively on the interplay between image acceleration towards the surface and repulsion by an ensemble of positive hole charges in the surface (“trampoline effect”). For ${\mathrm{Ne}}^{10+}$ we find that image acceleration is dominant and no collective backscattering high above the surface takes place. For grazing incidence, our simulation delineates the pathways to complete neutralization. In accordance with recent experimental observations, most ions are reflected as neutral or even as singly charged negative particles, irrespective of the charge state of the incoming ions.

The formalism of second-order many-body perturbation theory has been applied to investigate the electronic and geometric structures of neutral, cationic, and anionic ${\mathrm{Ag}}_n$ $(n=5\mathrm{}–9)$ clusters. Hay-Wadt relativistic effective core potentials replacing the 28 core electrons and a Gaussian basis set have been used. Full geometry optimizations of topologically different clusters and clusters belonging to different symmetry groups have been carried out. The neutral silver clusters prefer planar geometry up to $n=6\mathrm{}$ and the charged clusters prefer three-dimensional geometry from $n=6.\mathrm{}$ Binding energies, ionization potentials, electron affinities, and fragmentation energies of the optimized clusters have been compared with other experimental and theoretical results available in the literature. Based on different criteria, we predict the eight-atom silver cluster to be a magic-number cluster.

A femtosecond laser driven collisional Ne-like Ti x-ray laser at 32.6 nm is numerically investigated using a hydrodynamic code coupled with an atomic data package for a 100-μm-thick Ti planar target irradiated by a single or double prepulse followed by an intense femtosecond laser pulse. By using an optimized drive pulse configuration, a gain of $40{\mathrm{cm}}^{-1}$ can be generated from a $5\mathrm{mm}\times 50\mu \mathrm{m}$ line focus using only about 1 J pump energy.

We analyze a four-wave-mixing (FWM) scheme in a five-level atomic system based on electromagnetically induced transparency (EIT). We show that EIT suppresses both two-photon and three-photon absorptions in the FWM scheme and enables the four-wave mixing to proceed through real, resonant intermediate states without absorption loss. The scheme results in a several orders of magnitude increase in the FWM efficiency in comparison with a recent scheme [Phys. Rev. Lett. 88, 143902 (2002)] and may be used for generating short-wavelength radiation at low pump intensities.

We solve the Gross-Pitaevskii equation numerically for the collapse induced by a switch from positive to negative scattering lengths. We compare our results with experiments performed with Bose-Einstein condensates of ${}^{85}\mathrm{Rb},$ in which the scattering length was controlled using a Feshbach resonance. Building on previous theoretical work we identify quantitative differences between the predictions of mean-field theory and the results of the experiments. In addition to the previously reported difference between the predicted and observed critical atom number for collapse, we also find that the predicted collapse times systematically exceed those observed experimentally.

We calculate the optical properties of the quantum vacuum when a static magnetic field ${\mathbf{B}}_0$ and an electric field ${\mathbf{E}}_0$ are applied perpendicular to the direction of light propagation. We find a polarization-independent directional anisotropy in the refractive index, proportional to ${\mathbf{B}}_0\times {\mathbf{E}}_0.$ Our calculation confirms a general prediction that such an anisotropy should exist in any centrosymmetric medium, of which the quantum vacuum is a special case.

We investigate the entanglement characteristics of two general bimodal Bose-Einstein condensates—a pair of tunnel-coupled Bose-Einstein condensates and the atom-molecule Bose-Einstein condensate. We argue that the entanglement is only physically meaningful if the system is viewed as a bipartite system, where the subsystems are the two modes. The indistinguishibility of the particles in the condensate means that the atomic constituents are physically inaccessible and, thus, the degree of entanglement between individual particles, unlike the entanglement between the modes, is not experimentally relevant so long as the particles remain in the condensed state. We calculate the entanglement between the two modes for the exact ground state of the two bimodal condensates and consider the dynamics of the entanglement in the tunnel-coupled case.

We propose an implementation of an universal quantum cloning machine [UQCM, V. Buzek and M. Hillery, Phys. Rev. A 54, 1844 (1996)] in a cavity quantum electrodynamics experiment. This UQCM acts on the electronic states of atoms that interact with the electromagnetic field of a high-$Q$ cavity. We discuss here the specific case of the $\overrightarrow{1}2$ cloning process using either a one- or a two-cavity configuration.

We discuss the thermal and magnetic entanglement in the one-dimensional Kondo necklace model. First, we show how the entanglement naturally present at zero temperature is distributed among pairs of spins according to the strength of the two couplings of the chain, namely, the Kondo exchange interaction and the hopping energy. The effect of temperature and the presence of an external magnetic field are then investigated. We also discuss the adjustment of these variables in order to control the entanglement available in the system. In particular, it is indicated that there exists a critical magnetic field above which the entanglement undergoes a sharp variation, leading the ground state to a completely unentangled phase.

A simple algorithm is presented to derive accurately the exchange-correlation potential in the density functional theory (DFT) from the electron density. The method, which can be used with any physically acceptable density as input, is applied here to the densities in atoms obtained from high-level Green’s function calculations. The resulting potentials show the correct asymptotic behavior and the characteristic intershell peaks. We illustrate the possible use of these potentials in fitting procedures for new functionals, by investigating the HCTH functional [F. A. Hamprecht, A. J. Cohen, D. J. Tozer, and N. C. Handy, J. Chem. Phys. 109, 6264 (1998)]. The potentials derived from Green’s function one-body densities provide a microscopic foundation for present-day functionals in DFT, and may therefore be helpful in the ultimate goal of constructing functionals on a fully ab initio basis.

The resonant double photoionization of Ne has been studied via an electron-electron coincidence experiment. A complete set of measurements was performed at different mutual angles when the photoelectron and Auger-electron pairs have nearly equal energy. The overall experimental energy resolution, narrower than the natural linewidth of the intermediate state, has allowed us to observe interference effects due to the indistinguishability of the two electrons. The observed energy and angular distribution exhibit a strong interplay of postcollisional interaction and interference effects. Calculations based on the model developed by Sheinerman and Schmidt [J. Phys. B 30, 1677 (1997)] describe the main trend of the experimental observations.

Exact non-Born-Oppenheimer simulations of a one-dimensional model of one-electron ${\mathrm{H}}_2^{+}$ and linear ${\mathrm{H}}_3^{2+}$ in an intense short laser pulse are used to investigate the nonlinear multiphoton electron emission spectra, called above threshold ionization (ATI). Due to the rapid proton motion on near-femtosecond time scale, the ATI spectra are found to be produced at the critical internuclear distance $R_c\sim 7–8\mathrm{a}.\mathrm{u}.\mathrm{},$ leading to charge-resonance-enhanced ionization (CREI). As a consequence, maxima in the ATI spectra are displaced with respect to the similar H-atom spectra by a laser-induced Stark energy $E_M{R}_c/2,$ where $E_M$ is the maximum amplitude of the laser field. Highly oscillating ATI spectra occur, which are enhanced by the nuclear motion. These are interpreted as due to coherent excitations of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals, which are also responsible for CREI. Electron rescattering effects in the energy regions of ${10U}_p$ and ${8U}_p,$ where $U_p$ is the ponderomotive energy, are shown to be substantially reduced due to the rapid molecular dissociation and Coulomb explosion. Nevertheless, the ATI spectra in these energy regions reflect the different electron energies in the rescattering process. Fine structures of the ATI spectra are found to be enhanced by moving nuclei, reflecting the enhancement of resonant transitions by varying Franck-Condon factors during the nuclear motion.

We investigate schemes to dynamically create many-particle entangled states of a two-component Bose-Einstein condensate in a very short time proportional to $1/N,$ where N is the number of condensate particles. For small N we compare exact numerical calculations with analytical semiclassical estimates and find very good agreement for $N>~50.$ We also estimate the effect of decoherence on our scheme, study possible scenarios for measuring the entangled states, and investigate experimental imperfections.